Field of the Invention
The present invention relates to an apparatus for detecting the terminal end (hereinafter referred to as the "terminus") of cloth being sewn by a sewing machine and, more particularly, to an apparatus of this type which may be used in an industrial sewing machine or the like as an apparatus for detecting the cloth terminus or the cloth overlap.
An apparatus of this type is known from, for instance, the prior art disclosed in Japanese Patent Publication No. 2393/1986.
The prior art has an arrangement which consists of a combination of two cloth terminus sensors, sensors disposed in the needle tip detecting section and the pulley rotational angle detecting section, and an electronic arithmetic control section. In this arrangement, as shown in FIG. 7, two cloth terminus detecting sensors 21 and 22 are disposed upstream of the needle 20 in the direction in which the cloth is fed, and the cloth feed amount required for finally stopping a needle 20 correctly at a predetermined stop position is calculated by detecting the number of movements of the needle operation required to sew the cloth through a distance l1 between the two sensors.
The first cloth terminus detecting means 21 detects whether there is the cloth at a first cloth detecting position on the upstream side of the needle 20, while the second cloth terminus detecting means 22 detects whether there is the cloth between the first cloth detecting position and the needle 20. If it is assumed that the distance between the first and second detecting means 21 and 22 is represented by l1 while that between the second detecting means 22 and the needle 20 is represented by l2, it would be noted that the prior art encounters the following structural disadvantages:
(1) The distance which has to be sewn before the needle stops inevitably includes the distance l1 in addition to the distance l2. This means that the necessary sewing distance cannot be shorter than l1+l2.
(2) In actual operation, if the machine is to be stopped after a signal has been received from the first sensor, a certain time is required before it stops because of some mechanical reasons (such as those relating to the stopping performance of the motor). In addition, the distance l2 has to be set while taking into consideration the case where the amount by which the cloth is fed by the machine per movement of the needle operation is set at the maximum value (This amount of cloth feed, which can be set by the operator, will hereafter be called the "feed amount"). Accordingly, these factors inevitably increase the necessary sewing distance l1+l2. In particular, when the feed amount is set at a small value, the stoppage error will be relatively large.
An additional description will be given concerning the distance 2. This distance must satisfy the following relationship if, for instance, four movements of the needle operation are necessary before the machine stops in stopping the motor in response to a signal output from the second sensor 22 and, simultaneously, the maximum value of the feed amount is 4 mm:
Because 4 mm.times.4=16 mm, EQU l2&gt;16 mm
(the distance corresponding to this 16 mm will hereafter be referred to as a "stop enable distance").